IPhone 3G Screen Help ?

My iPhone 3G is acting up. The screen is completely frozen. I will push the home button to use my iPhone, but it is frozen where it says "slide to unlock", but when i was testing to see if it actually was frozen (was going to restart it by holding the "Home" button and the "Lock" button). But when i was going to do that, i can perfectly hit "Cancel" right before it restarts. I don't understand that.